There are certain types of transmitted digital data signals which do not readily lend themselves to conventional phase-locked loop detection techniques. In the customary phase-locked loop detector, timing of the sampling clock is adjusted for optimum detection of the symbols by synchronizing the sampling to the symbol rate of the incoming signal.
In certain cases, such as digital processing of unsynchronized frequency-multiplexed signals, the sampling cannot be synchronized to the incoming signal.
Systems have been proposed, when it is not possible to alter the sampling clock, for interpolating among the non-synchronized samples so as to produce the correct strobe values at the output of a digital processor which are the same as the strobe values which would occur if the original sampling had been synchronized to the symbol (see F. M. Gardner, "Interpolation in Digital Modems--Part I: Fundamentals", IEEE Transactions on Communications, Vol. 41, No. 3, pp 501-507, March 1993 and "Interpolation in Digital Modems--Part II" IEEE Transactions on Communications, Vol 41, No. 6, pp 998-1008, June 1993).
While the interpolation system proposed by Gardner is able to be adjusted within a limited range, the co-existence of a wide variety of data transmission standards for high speed data services, such as digital television signal transmission, imposes variability requirements beyond those contemplated in Gardner's proposals.
In a somewhat analogous situation, involving adaptable modems, an approach was taken by two of the present inventors, Bao and Poon, along with co-workers Oshiki Mizutani and Hiroyki Nakayama of Japan, to provide a reconfigurable "Universal Modem for Digital Video, Audio and Data Communications" (see U.S. Pat. No. 5,940,438 issued Aug. 17, 1999). In that universal modem arrangement, the modem is arranged to be software reconfigurable to accommodate different data rates and different modulation schemes.
In a related manner, the present invention is directed to providing a program (software) controlled, reconfigurable symbol timing recovery system with non-synchronized sampling. The system is particularly suitable for processing high definition digital television signals with various standards such as Vestigial Side Band Modulation (VSB--over the air signals), Quadrature Amplitude Modulation (QAM--cable signals) and Quadrature Phase Shift Keying (QPSK--satellite signals).